A technology of transferring an undulated pattern of a nanometer size (0.001 to 1 μm) (hereinafter, also referred to as a “nanostructure”) of a mold to a resin material formed on a substrate by pressing them together, so-called nanoimprint technology, has attracted attention recently. Applications of the nanoimprint technology to optical materials, microfabrication of ICs, substrates for clinical laboratory test, and the like are now being researched. This technology has been achieved through development of a hot embossing technology, which is known in preparation of optical discs, and S. Y. Chue et al. proved in 1995 that this technology allows embossing of features as small as 10 nm.
Nano-scale fabrication by conventional photolithography involves exposure through a mask, and this exposure causes diffraction phenomenon. In order to eliminate poor resolution resulting from this phenomenon, light at a short wavelength is needed for the exposure. This deficiency of the nano-scale fabrication is followed by a need of a more complicated device, and an increase in costs, for example. The nanoimprint technology is free from the above-mentioned deficiencies because in this technology, nanometer-sized patterns can be easily formed by embossing. Further, through this technology, optical components with a nanometer-sized structure can be mass-produced inexpensively. Thus, the nanoimprint technology has attracted attention.
Thermal nanoimprint and UV nanoimprint are known as the nanoimprint technology. According to the UV nanoimprint, for example, a mold with nanostructures is pressed against a UV-curable resin thin film formed on a substrate, and the film is irradiated with UV light to yield a thin film with nanostructures (hereinafter, also referred to as a nanoimprinted sheet) in the inverse shape of the mold. When these nanoimprint technologies are used in a research phase, preparation of nanoimprinted sheets generally involves use of a plate mold and batch process. In order to mass-produce the nanoimprinted sheets at a low cost by the nanoimprint technology, roll-to-roll process is preferable to batch process. This is because the roll-to-roll process allows continuous production of the nanoimprinted sheets by a transfer roller having an outer circumference surface on which nanostructures are formed.
Briefly, the roll-to-roll process is mentioned below. With respect to a nanoimprint technology involving the roll-to-roll process, for example, Patent Document 1 discloses a method including transferring a pattern of a small transfer roller 191 onto a UV-curable resin coated on a large transfer roller 192 while the pattern is extended by sequentially moving the transfer roller 191 laterally as shown in FIG. 13. However, in this method, the pattern is formed on the resin on the large transfer roller 192 by extending the pattern of the small transfer roller 191. Therefore, the pattern of the formed nanostructures usually has a seam. Thus, this method is not suitably used for forming nanostructures over a workpiece with a width larger than that of the small transfer roller 191.
With respect to rollers used in technologies other than the nanoimprint technology, which are not the transfer rollers used for the roll-to-roll process, for example, Patent Documents 2 and 3 disclose a method of producing a roller with an undulated pattern directly formed thereon. When this method is adapted in the nanoimprint technology, a transfer roller with nanostructures needs to be equipped with, for example, a bearing mechanism for coupling the transfer roller with a nanoimprinter. This leads to an increase in costs on the transfer roller, which is a problem in view of mass-production.
Further, for example, Patent Document 4 discloses, in FIG. 7, a method of mounting a cylindrical member having an undulated pattern on a roller. According to this method, it is difficult to form a continuous nanopattern by bending the member around the outer circumference of the roller, and as a result, the transfer roller has a seam in the nanopattern.
Next, optical materials with nanostructures are mentioned. In optical materials, a “moth-eye structure” is known as one type of the nanostructure. The moth-eye structure includes, for example, a large number of conical protrusions of a size much smaller than visible light. Optical elements with such a moth-eye structure include one having a moth-eye structure formed on a transparent substrate surface. In this moth-eye structure, the size of the protrusion is much smaller than a visible light wavelength, and therefore, visible light entering the transparent substrate surface recognizes that a refractive index continuously changes from air to the transparent substrate because of the protrusion, and as a result, it does not recognize the air/transparent substrate interface as a refractive index mismatch interface. Thus, light reflection on the transparent substrate surface can be markedly decreased, for example, as disclosed in Patent Documents 6 to 9.
In a technology of producing optical materials with such a nanostructure, for example, Patent Documents 5 to 8 disclose a method of using an aluminum substrate having a surface with nanometer-sized cavities formed thereon by anodization. According to this method, nanometer-sized structures can be formed on the surface microscopically in a random placement and macroscopically in a uniform distribution. Specifically, this method is used for producing the transfer roller, whereby seamless nanostructures, which are needed for continuous production, can be formed on a surface of a columnar or cylindrical mold roller (for example, see FIG. 19 of Patent Document 8).
When such a transfer roller with the nanostructures is adapted for use in a nanoimprinter used for the roll-to-roll process, the transfer roller is not able to be used permanently and needs to be replaced after being used for a certain period. Therefore the transfer roller is strongly desired to be inexpensive.
It is effective in meeting this demand to use a cylindrical transfer roller and to make the structure of such a replaceable transfer roller simple. However, the use of the cylindrical transfer roller has the following disadvantages.
The transfer roller is required to be mounted on a nanoimprinter with high control accuracy for its position and direction because the transfer roller transfers the nanostructure onto a surface of a workpiece sheet while uniformly pressing the sheet. When a cylindrical transfer roller is used, a member serving as the axis of rotation for the roller is needed. In such a case, on the inner circumference side of the cylindrical transfer roller, an axis shaft having the rotation axis coincident with that of the transfer roller can be mounted. However, it makes the replacement of the transfer roller complicated to mount the transfer roller on the imprinter with high accuracy so that its rotation axis is coincident with that of the shaft. In view of this, the transfer roller is required to be more easily replaced.
Further, a treatment (pre-transfer treatment) where a workpiece sheet before being coated with a resin is made strained and made to travel is performed before the transfer, thereby ensuring that the sheet can travel in the imprinter smoothly. This pre-transfer treatment prevents the workpiece sheet, which is to yield a product, from getting twisted or wrinkled. This pre-transfer treatment possibly causes the following problems, for example. When a foreign substance and the like is caught between the workpiece sheet and the transfer roller, for example, the transfer roller surface is possibly damaged, and the pattern for forming nanostructures on the surface might be deformed, or a demolding agent coated on the transfer roller is possibly removed. Particularly the nanometer-sized pattern prepared by the anodization is easily deformed by a local intense pressure that is generated by the foreign substance and the like because the base material is aluminum. This possibly leads to poor transfer onto the workpiece sheet, a reduction in yield, and an increase in costs arising from the replacement of the transfer roller. Particularly when the demolding agent is removed from the transfer roller surface, a resin material forming the nanostructure adheres to the transfer roller, and this causes the poor transfer onto the sheet. Therefore, it has been desired that the transfer surface of the transfer roller is protected against the damages.